From the AS website
The following exceptions to the checked baggage charges apply to all customers:
Strollers and car seats may be checked without charge.
Wheelchairs and other assistive devices may be checked without charge.
Customers making international connections to other airlines within twelve hours of arriving in your connection city, may follow the baggage allowance and weight restrictions of the international airline - provided a ticketing and baggage agreement is in place between Alaska Airlines/Horizon Air and your other carrier. Please contact Alaska Airlines Reservations at 1-800-252-7522 (TTY/TDD Users: 1-800-682-2221) or Horizon Air Reservations at 1-800-547-9308 for more information.
Traveling out of Santa Rosa, customers are allowed to check one box of wine (if properly packaged) for no additional fees.